(1) Field of the Invention
The present invention relates to a blade with reduced vulnerability, i.e. a blade having optimized tolerance to damage. For example, the invention relates to a blade for a rotor, in particular a rotorcraft rotor.
(2) Description of Related Art
Conventionally, a blade comprises a rigid working shell contributing to the structural strength of the blade, the working shell sometimes being referred to as a “skin”, or an “outer skin”, or indeed a “working skin”.
The working shell has a suction-side wall and a pressure-side wall that extend from a leading-edge first zone to a trailing-edge second zone. In a blade made of composite materials, the working shell may have a suction-side wall and a pressure-side wall that are secured to each other, each including at least one layer of fabric made of high-strength fibers united by a matrix. The suction-side wall and the pressure-side wall may be made separately and then connected together, or they may be made together simultaneously, e.g. by winding.
The working shell is hollow, generally so as to enable it to receive both at least one spar serving in particular to take up the centrifugal forces that act on the blade while it is in rotation, and also at least one filler element. Each spar may be made of composite material, e.g. being made using fibers that are united by a matrix.
Furthermore, it is common practice to provide the trailing-edge zone of the working shell with an edge strip in order to reinforce the blade.
Finally, the inner space of the working shell that is not occupied by a spar is filled with at least one lightweight filler element, in particular to avoid the outer shell deforming under the effect of external pressure, which pressure varies in particular as a function of altitude.
Furthermore, the blade may include at least one internal link member connecting together the pressure-side wall and the suction-side wall. The internal link member then extends in the span direction of the blade inside the working shell between the leading-edge zone and the trailing-edge zone of said blade so as to connect together the suction-side wall and the pressure-side wall. Such a link member may be a spar or a rib.
Under such circumstances, depending on the embodiment and by way of example, a blade may include a solid leading-edge spar fitted with a counterweight for centering the blade together with an internal link member, the leading-edge spar extending in the span direction of the blade inside the working shell in the leading-edge zone of said blade.
Furthermore, a first filler element is arranged inside the working shell between the leading-edge spar and the internal link member, and a second filler element is arranged inside the working shell between the internal link member and an edge strip arranged in the trailing-edge zone of said working shell.
Furthermore, it is common practice to cover the outside of the leading-edge zone of the working shell with a protective fairing commonly referred to as a “leading edge” or as a “leading-edge fairing” given its location.
A fairing serves in particular to protect the blade against impacts.
Document FR 2 748 719 describes a blade having a structure of that type.
Furthermore, it should be observed that certain blades include deicing and/or anti-icing devices. Such a deicing device is sometimes provided with deicing means connected to an electrical power network via wired power supply means and wired electrical return means between the fairing and the working shell of the blade.
In the event of an impact with a foreign body, the fairing may suffer residual deformation. Under such circumstances, the working shell is sometimes also deformed and becomes locally separated from the spar and the filler means.
Furthermore, when the leading-edge spar is made of composite material, the matrix of the spar becomes cracked, thereby leading to delamination and consequently to portions of fiber becoming separated from the resin, for example.
A serious impact thus severely damages a rotor blade.
Furthermore, it should be observed that it is difficult to implement the electrical return means of a deicing device, with it being possible for the electrical return means to be damaged by the heat given off by the deicing means when in operation.
The technological background includes document FR 2 617 119.
Document FR 2 617 119 describes a blade of composite material having a structural core, the structural core having a working shell filled in particular by means of a leading-edge spar, a link member of the central spar type, and filler elements.
Furthermore, the blade is provided with a covering skin surrounding the structural core. The covering skin possesses a rigid wall and a layer of non-structural shaping material that surrounds the structural core, said layer being interposed in practice between the structural core and the rigid wall.
Furthermore, document FR 2 689 565 departs from the invention by presenting a composite structure that is provided with a main core covered using a complex surface skin, which skin comprises both an elastomer member and a rigid protective layer.
Documents U.S. Pat. No. 2,648,388, US 2008/107540, and U.S. Pat. No. 3,765,124 are also known.